Yes I have done it with excellent results. Forget about using an EI type, only toroidals will work well. Mine is in a 807 ultra linear arrangement (using the 110V tap on the dual primary for the UL) this works because the 807 draws little screen current as it is a specially designed tetrode. Try to have no unused secondary windings as they will spike the response. You need to specify at least 4x the required output power as the VA rating to get decent bass, though in parafeed you can boost the bass by correct specification of the parafeed cap (a black art indeed). I used plate to plate feedback to lower the effective output impedence of the 807's so that they were better able to drive the inductance and capacitance of the toroidals, this was based on the RH807 design, though the feedback resistor was increased from 100K to 130K to stop suggested bass boost. I used a 110+110 : 18V+18V with the secondaries in parallel. I unwound about two thirds of the secondaries to get the correct ratio.
Don't believe anyone who tells you it won't work or will sound rubbish - they are wrong - wrong - wrong !!!

I am currently experiementing with toroidals as PP outputs as well.

If you are at all interested I can supply the schematic of the design.

I tried a parafeed with toroidal too.
I built a parafeed with 2 KT88 connected in parallel, loaded with a solid state CCS (150mA).
I used a 150W, 230/9V main toroidal transformer (with a 4 Ohm load this is a 2500 Ohm primary inductance).
It sounded GREAT!

I'm also thinking of using toroidals for PP output, what I've on my bench is an EL36 triode PP amp. I will experiment and make measurements, and post here and on my site.

Shoog, and other people experienced with this: I live in 220V country, I need 8k load, so I need a 7V secondary. Since 7V isn't standard, and most of the toroids have 2 secondaries (or one centertapped), what is the best way to go? 7Vct doesn't exist, but 2 x 15V (to be paralleled) is a standard value, am I right?

What's the best in terms of audio performance, put secondaries in series or in parallel (given the same impedance ratio)?

The main voltage standard in Italy is not anymore 220V, but 230V.
The main voltage value has been increased some years ago.
For this reason the primary/secondary ratio of transformers has been slighty modified (230/15V is different than 220/15V).
If you parallel 2 secondary eindings at 15V, you still have a 15V secondary, but with a double current capability: this means the primary inductance will not change at all.
As you know, the primary inductance can be calculated with a the formula:

If you want to have a 8K load, with a 230V primary winding, you need a secondary winding of:

4 Ohm = 5V secondary
8 Ohm = 7V secondary (your request in your post)

The best way to obtain it is to completely unwound the secondary winding, counting the turns while unwounding it.
At the end, you can reuse the wire to wound a different secondary with the right turns.

For example, let's suppose you have a 230/15 toroidal transformer.
Unwounding the secondary gives you a 200 secondary turns count.
At this point you have a 15V/200 = 0,075 V per turn.
If you need a 5V secondary, simply calculate how many turns you need with:
5V secondary = 5V / 0.075V_per_turn = 66 turns
7V secondary = 7V / 0.075V_per_turn = 93 turns

Hope this is clear.
If you need, please contact me, I can explain everything better in our italian language.

I do not know of any company selling a toroidal transformer with a 5 or 7V standard output.
your choices are:

1. To find a company that would prepare a customized toroidal transformer for you, with a primary of 230V and a secondary of 5V or 7V.
This is really hard, cause such companies are really rare.

2. Unwind and rewind your transformer on your own.
I sugest this second alternative, it is really really easy.
You have only to remove the secondary winding (that is the first on top) and then rewind a new one, using the same wire, all around the transformer (spreading it all along the toro).
My sugestion is to wind two or three secondary, with exactly the same turns, and then connect them in parallel.

Speaking about current: if you have more than one secondary winding, place them in parallel, the total resistance (in series with the loudspeaker) will be reduced and, thus, the dynamic should encrease.

I will report on my finding on PP when I have some satisfactory results.

An alternative to completely unwinding the secondary is to partially unwind as I did.
The procedure is;
Hook transformer up to a AC voltage source
Take a voltage reading on the unmodified secondaries.
Unwind 10 turns from each of the secondaries
Take another voltage reading.
Decide on what reduction you are seeking and divide it by the voltage drop produced by by removing the 10 turns. Multiply this number by 10 to get the number of turns you need to remove.
The secondary will be loose so wind insulating tape over the whole.
Start with a 10V secondary.
It sounds frightening but is surprisingly easy - if a little tedious. Its a little more complicated than it sounds because the winding will tuck under itself in places.
If you want 10Watts of output go for a 120VA transformer.